The observed labeling index (TI) following administration of tritiated thymidine is a function of duration of emulsion exposure and a TI curve results from a graph of observed TI versus duration of exposure. These curves have been shown to be significantly different for seven murine tumors and appear to describe the distribution of 3H atoms incorporated into cells in the tumors. The label found in all cells scored as labeled is specifically removed by DNase, and the TI curves for several murine tumors are not modified by route of injection, or pretreatment with oral thymidine or 5-fluorouracil. Pretreatment with 3.0 mg/g body weight hydroxyurea reduces the intensity of label but not the maximum observed TI. Surgical removal of a primary Lewis lung tumor results in an increase in the observed TI at every emulsion exposure for residual lung metastases and is consistent with an increase in sensitivity. The intensity of label and the distribution of 3H obtained from a computer model to fit the TI curve correlate with the response to chemotherapy. The more sensitive tumors appear to contain a higher fraction of intensely labeled cells as well as a broad distribution of 3H labeled cells that includes lightly labeled cells. Studies are underway to attempt to modify the curves and to determine the factor(s) responsible for the distribution of 3H.